Stroke is the most common cause of permanent disability, and remains the third leading cause of death in industrialized countries. Carotid endarterectomy (CEA) and stenting (CAS) have shown to be effective in stroke prevention. However, only clinically evident embolization has been evaluated as an outcome measure; subclinical microemboli have not been fully assessed. Several studies have confirmed that microemboli demonstrated on diffusion-weighted MRI (DWI) during CAS are common despite absence of clinical symptoms. To date, the clinical significance of microemboli and their long-term effects on cognitive function are largely unknown. To elucidate these important issues, we propose an intense longitudinal investigation using a multidisciplinary team approach at two academic centers. Our central hypothesis is that subclinical microembolization moderates the degree of cognitive changes following CAS and that development of microemboli themselves is associated with patients- and procedure- related factors. Three specific aims are proposed to address this urgent issue: Specific Aim 1: Determine the clinical significance of the development of DWI abnormalities for short- and long-term changes in cognitive function in patients with subclinical microemboli; Specific Aim 2: Evaluate patient risk factors for subclinical microembolization following CAS; and Specific Aim 3: Identify technical factors contributing to procedure-related subclinical microemboli during CAS. The investigators and collaborators have complementary expertise and a track record of clinical research in the areas of carotid disease (WZ, RLD, MGD), brain imaging (BL, AR, ZG), and cognitive function (AR, JY, LL). Statistical analyses will be conducted in consultation with Dr. Lazzeroni, an experienced biostatistician. Through this proposal and collaborative efforts of multidisciplinary team experts, we hope to understand three key questions: Are subclinical microemboli clinically relevant? Do they affect long-term neurocognitive function? What are the risk factors for microemboli? Understanding these fundamental issues is essential to optimizing our strategy in stroke prevention through proper patient selection, a better treatment approach, and possible refinement in interventional techniques. Reduction of potentially microemboli-associated cognitive impairment will also have a significant impact on future public health by decreasing vascular-related dementia. Ongoing debates regarding the treatment guideline for CAS raise the urgency of this study. The proposal may change our current clinical practice guideline by providing a better outcome measure for carotid interventions. Regardless of the cognitive effects of microemboli, risk factor stratification represents a unique opportunity for quality improvement in carotid interventions.